Positive active mass

Typical pore size distributions result in mean pore diameters of around 15 //m. Even long and intensive efforts did not succeed in decreasing this value decisively in order to enable production of micropo-rous pocketing material resistant to penetration [65, 66], In practice PVC separators prove themselves in starter batteries in climatically warmer areas, where the battery life is however noticeably reduced because of increased corrosion rates at elevated temperature and vibration due to the road condition. The failure modes are similar for all leaf separator versions shedding of positive active mass fills the mud room at the bottom of the container and leads to bottom shorts there, unless — which is the normal case — the grids of the positive electrodes are totally corroded beforehand. 

The basic materials are sufficiently stable in sulfuric acid not to require the expensive phenolic resin impregnation. Traces of adhesive are applied to hold the glass mat in order to achieve the total thickness. This separation system may be expensive to manufacture, a fact certainly largely balanced by savings in positive active mass, but it also has some indisputable advantages. 

One of possible ways of improving the specific characteristics of LIB electrodes is the use of thermally expanded graphites (TEG) as electrically conductive additives of positive active mass. The efficiency of TEG s use may be illustrated by analyzing a simple theoretical model proposed by us AM - electrically conductive additive (Figure 4). 

Therefore, passivation of the positive electrode by poorly conducting PbS04 can be reduced [348]. The porosity is important because it enables the expansion during the solid phase volume increase, which accompanies the transformation of Pb02 to PbS04. In the most popular construction, the electrode paste material (mixture of metallic lead with lead oxides) is held in a framework composed of lead alloys with additions of tin, antimony, selenium, and calcium [348]. Antimony improves the mechanical stability however, it increases the resistance and facilitates the selfdischarge of the battery. Better results are obtained for low antimony content and/or for lead-calcium alloys [203]. Methods of positive electrodes improvement, from the point of view of lead oxide technology have been discussed [350]. Influence of different factors on life cycle, nature, and composition of the positive active mass has been studied by Pavlov with coworkers [200, 351, 352]. 

Under discharge, the oxidation of MH is associated with the reduction of NiOOH into Ni(OH)2 in the positive active mass. When the external circuit is open (no current), the active mass (Reaction 1.1) confers to the negative current collector, a potential versus the standard hydrogen electrode (SHE) according to the Nemst law ... 

An important result of these studies was the discovery that a lack of antimony has not only an effect on the grid/positive-material interface, but also on the whole crystalline structure of the positive active mass [43] and on the so-called mass softening process. Nowadays deleterious effects that are promoted by using grids without antimony are collectively termed premature capacity loss (PCL). There has been much research effort worldwide to overcome the PCL problem [47]. 

With the introduction of lead—calcium alloys for the plate grids, however, the life of the battery on deep discharge cycling declined dramatically to 20—25 cycles. This phenomenon was first called Sb-free effect and later premature capacity loss (PCL effect) [20]. It was established that the PCL effect was a result of certain processes that proceed at the positive battery plate, more precisely at the interface grid/positive active mass (PCL-1 effect) [21] and/or in the positive active mass volume (PCL-2 effect) [22]. 

Experimental values for the density of Pb02 are given in Table 2.6 [26], It can be seen that the density of a-PbOa is somewhat higher than that of p-Pb02- The data in the table evidence also that Pb02 in the active mass has a substantially lower density than that of the crystal deposit formed by electrochemical oxidation in the lead-ion-containing solutions. Micka et al. proposed the presence of amorphous PbCh in the active mass [66]. The formation of a third amorphous modification of Pb02 in the positive active mass of lead—acid batteries has been established by many researchers [67—70]. 

The above-described structure of the lead dioxide particles is likely to exert an influence on the electrochemical and chemical processes that proceed in the positive active mass on charge and discharge, as well as on the behaviour of the positive plates in sulfuric acid solution. 

PAM samples with H2SO4 solution were X-rayed. After 160 min stay in the solution the positive active mass was washed and dried. The intensities of the diffraction lines after this treatment are presented on the right-hand side of the figure. 

The ratio between gel and crystal zones in the Pb02 particles depends also on the presence of Li" " ions, too. The ions formed as a result of oxidation of the grid alloy would also have an impact on the electrochemical behaviour and the capacity of PAM, by affecting the crystal/gel zones equiUbrium. Thus, the positive active mass is an open system. 

Proton—Electron Mechanism of Discharge of the Positive Active Mass... 

Specific capacity of the positive active mass vs discharge current density for three types of batteries SGTP, SLI and conventional cylindrical tubular plates [103]. 

Positive plates need much more time to form than negatives. The reason for this is the dielectric behaviour of the cured positive paste. Oxidation of the bivalent lead compounds in the paste and formation of the Pb02 positive active mass passes through a number of chemical reactions, some of which proceed at a low rate, which retards the technological process of formation of the positive plate. In an attempt to accelerate the formation process, additives to the positive paste have been looked for, which are characterised by electro-conductive properties and stability in sulfuric acid. These additives create an electro-conductive network in the paste and the process of oxidation proceeds simultaneously within a large paste volume, thus accelerating plate formation. 

Table 7.4 Materials proposed to enhance the conductivity of the positive active mass [52].

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